Reclamation Information Sharing Environment (RISE)

The Bureau of Reclamation has been asked to perform a thorough evaluation of the Bighorn River from Yellowtail Dam to the St. Xavier Bridge near Ft. Smith, MT. The investigation was divided into two parts, a geomorphic investigation (companion document), completed by Godaire in 2010, and a hydraulics and sediment transport investigation (this document). The primary focus of the overall investigation was the loss of side channel connectivity at frequent discharges. Over the past decade, and perhaps a bit longer, the entrance to many side channels have begun to aggrade due to the infrequency of high discharges capable of transporting sediment through the side channels and preventing the encroachment of vegetation. This report details the hydraulics of the Bighorn River within the study reach and evaluates potential solutions to stop, or reverse, the aggradational trend occurring in the entrances of many side channels. Sediment transport was evaluated with a two dimensional (2D) hydraulic model in a static condition, whereby the bed did not undergo deformation and discharges were steady. Conditions of sediment transport was evaluated across the spectrum of bed motion, from zero transport to vigorous transport. The numerical modeling of sediment motion was coupled with observation of sediment transport using particle tracers, resulting in a validation of sediment results provided by the model.